1. Field of the Invention
This invention relates to a method of producing amorphous thin films. More particularly, it relates to a method of sintering particulate material with a laser beam to produce amorphous optical thin films.
2. Disclosures of Interest
In the manufacture of lasers, semiconductors and other electronic or electro-optic devices, it is sometimes desirable to coat a substrate with a thin film of glass which has a different index of refraction than the substrate, and may be reflective or anti-reflective. For example, high peak-power laser systems, such as those used for fusion energy research, require reflective and anti-reflective optical coatings that are capable of handling pulsed laser powers up to 4.times.10.sup.9 W/cm.sup.2 (40 J/cm.sup.2 in 10-ns) at 1060 nm without damage.
The thin films can be deposited on the substrate by a number of means, including electron beam evaporation, reactive sputtering and plasma CVD deposition.
The thin films could be produced by heating particulate glass forming material above the liquidus, pouring the liquid material over the substrate, and then allowing it to solidify. This is undesirable for a number of reasons. For example, the thin films which are poured cannot be made sufficiently flat to satisfy optical requirements of the coatings. Moreover, silica is the preferred thin film material and to heat it to the point where it becomes pourable requires that it be heated to a temperature of at least 2000.degree. C. This requires considerable investment in equipment and energy costs. Glass forming materials which liquefy at temperatures lower than silica can be used, however, stress problems are encountered when they cool, thus requiring extra processing steps.
One solution to this problem is to sinter the materials into glass in situ on the substrate. The technique of sintering is described in a number of references. One in particular is U.S. Pat. No. 4,419,115, Johnson. In this patent there is described a method of forming a high silica, substantially alkali free glass by sintering an appropriate crystalline starting material at temperatures below the liquidus. The sintering is accomplished by forming a gel of the substance, drying the gel, then heating it to a temperature below the liquidus, but sufficient to cause viscous flow of the crystalline material in the gel.
The Johnson patent, however, is concerned with the sintering of bulk material, not thin films. The Johnson technique is not adaptable to thin films because the kinetics of sintering of bulk films is different than that of thin films. There is no correlation between the two.
What is needed in the art is a method of sintering thin films in situ on a substrate. The inventor has discovered such a technique utilizing a laser.
Laser-substrate interaction has been studied. Temple et al. (Temple, P.A., Lowdermilk, W.H. and Milam, D. Appl. Opt., 21, 3249, 1982) have described laser damage threshold increases for bare silica surfaces polished by CO.sub.2 laser light. Strutt and Yi (Yi, J.J.L. and P.R. Strutt, Adv. Fusion of glass, 49.1, 1988) have prepared novel glassy material using rapid heating and cooling by CO.sub.2 lasers. Haisma (Haisma. J., Appl. Opt., 24(16), 2666, 1985) has reported some general work in sintering TiO.sub.2 layers for use as reflectors. Stewart et al. (Stewart, A. F., A. H. Guenther and F. E. Domann, AFWL/PA 87-830, submitted to Boulder Damage Symposium, The Properties of Laser Annealed Dielectric Films, 1987) have used CO.sub.2 lasers as a post deposition annealing step on dielectrics prepared by e-beam evaporation, reactive sputtering and sol-gel. CO.sub.2 lasers have also been used to anneal SiON thin films on Si wafers (Lam, D.K.W., Appl. Optics, 23(16), 2744, 1984).
None of the foregoing references describe the method of the present invention.